The invention relates to photon detectors, particularly to x-ray photon detection, and more particularly to an x-ray detector which involves the conversion of x-ray photons into electrons (photoelectrons) and subsequent amplification of these photoelectrons via generation of electron avalanches.
Photon detectors operate by converting photons into electronic signals that can be processed into pulses or images. These include devices such as photodiodes, photomultiplier tubes, vidicons, charged-coupled devices (CCD's) and silicon detectors doped with impurities such as lithium, etc. All photon detectors are characterized by their sensitivity to photons as a function of photon energy, their ability to amplify incident photons into large electrical signals proportional to the incident photon intensity (gain), their ability to distinguish fine detail in an image (position resolution), their temporal response to incident photons (time resolution), and their inherent noise level (dark current). These prior devices are exemplified by U.S. Pat. Nos. 5,032,729 issued Jul. 16, 1991 to G. Charpak; No. 4,999,500 issued Mar. 12, 1991 to A. Breskin et al.; No. 4,896,041 issued Jan. 23, 1990 to H. Vlasbloem et al.; No. 4,859,855 issued Aug. 22, 1989 to H. Vlasbloem; No. 4,686,368 issued Aug. 11, 1987 to H. L. Anderson et al.; and No. 4,431,921 issued Feb. 14, 1984 to H. A. A. W. Filthub.
There are many applications where high efficiency and high image resolution of x-rays are important, such as x-ray microscopy and holography using incoherent and coherent sources of gamma rays and x-rays. A new field is microscopy of biological samples using x-rays and is reliant on new sources of intense x-rays available at synchrotron light sources. Traditional imaging has been accomplished using Si(Li) detectors and x-ray pinhole cameras or grazing incidence or refractive (Fresnel) optics. In a pinhole camera, the image is built up slowly by counting photons as the object is scanned across a fine x-ray beam formed by the pinhole. The process is complex and time consuming. Certain applications call for reasonable quantum efficiency of the detector at room temperature, which cannot be provided by the Si(Li) detectors which require bulky liquid nitrogen cooling systems for efficient operation.
Thus, there is a need in the field of photon detectors, for a detector capable of exhibiting photon sensitivity over a wide range of energies, has fast time response thus allowing operation at a frequency up to the GigaHertz level, provides adjustment gain, exhibits low noise, and has the capability to operate with reasonable quantum efficiency at room temperature. The present invention fills the above-mentioned need and provides a replacement for present-day x-ray photodetectors, such as the Si(Li) detectors, while exhibiting the fast time response typical of photomultiplier tubes, provides adjustable gain of up to about 10.sup.9, exhibits low noise typical of photomultiplier tubes, while operating efficiently at room temperature. Also, the present invention has the capability to perform as a position sensitive detector with position resolution similar to a CCD or Vidicon, but with a factor of 1000 times the speed of readout. The speed of the invention allows for unprecedented dynamic range of visual information, allowing for the detection of photons with high fidelity in conditions that would normally saturate CCD detector pixels. Unlike CCD detectors, the present invention is not subject to radiation damage.